Polypeptide compounds and other amino acid derivatives useful as antihypertensive agents and angiotensin converting enzyme inhibitors are well known classes of synthetic compounds. See for example, Bellini et al., in U.S. Pat. No. 4,216,209; Bumpus in U.S. Pat. No. 3,976,770; Ondetti et al. in U.S. Pat. No. 4,173,704; and Wille in U.S. Pat. No. 3,915,948. A natural product polypeptide with vasoactive and hypotensive properties has been isolated from the lungs of animals, especially hogs; see, Said et al., U.S. Pat. No. 4,220,642.